1. Field of the Invention
The present invention relates to a method of driving a liquid crystal display panel of the active matrix type which uses non-linear resistance elements as switching elements. In particular, the invention relates to a method of driving a liquid crystal display panel having non-linear resistance elements that exhibit asymmetric non-linear characteristics depending upon the polarity of a voltage applied to the elements.
2. Description of the Related Art
The liquid crystal display panels are becoming large, and the liquid crystal display panels of a simple matrix constitution which employ multiplex drive systems have a problem of a decrease in contrast with an increase in the rate of time division, making it difficult to obtain a sufficient degree of contrast in the case when they have 200 or more scanning lines. In order to eliminate the above defect, therefore, there has been employed a liquid crystal display panel of the active matrix type in which the individual liquid crystal pixels are provided with a switching element. The liquid crystal display panels of the active matrix type can roughly be divided into those of the three-terminal type which use thin-film transistors and those of the two-terminal type which use non-linear resistance elements. From the standpoint of construction and fabrication, however, the panels of the two-terminal type are superior. The panels of the two-terminal type include those of the diode type, varistor type, MIM (metal-insulator-metal) type and the like types. Among them, however, the panel of the MIM type is particularly simple in construction and can be fabricated using a reduced number of steps.
FIG. 10 shows a constitution of a liquid crystal display panel which employs non-linear resistance elements. Scanning electrodes S1 to SN and signal electrodes D1 to DN are provided on the opposing surfaces of two pieces of glass substrate. A display pixel consisting of a non-linear resistance element 41 and a liquid crystal pixel 42 is formed at each intersecting portion of the scanning electrode and the signal electrode. When a drive voltage is applied to turn the liquid crystal pixel 42 on, the non-linear resistance element exhibits a small resistance and the liquid crystal pixel is turned on with a small time constant. When the drive voltage is turned off, the non-linear resistance element exhibits a large resistance and the electric discharge takes place with a large time constant. The result therefore is an increase in the ratio of effective values of voltages applied to the liquid crystals when they are to be turned on and off, making it possible to carry out the multiplex driving while maintaining a high pixel density.
Some non-linear resistance elements exhibit asymmetric non-linear characteristics depending upon the polarity of the applied voltage. That is, referring to FIG. 2 which shows the transmission factor with respect to the write voltage, the positive-side characteristics and the negative-side characteristics are asymmetrical to each other and electrical-optic due to the asymmetric characteristics of the non-linear resistance element. Here, the positive side stands for the case where a positive voltage is applied to the non-linear resistance element when the display pixel is regarded to be an equivalent circuit in which the non-linear resistance element and the liquid crystal pixel are connected in series, and the negative side stands for the case where a negative voltage is applied thereto. FIG. 11 shows voltage-current characteristics wherein large asymmetric characteristics are exhibited with respect to the polarity of the applied voltage. The curve A represents element characteristics of the positive side and the curve B represents element characteristics of the negative side. When the liquid crystal display panel is to be multiplex-driven, in general, the voltage applied to the liquid crystal pixel is inverted for every field (period from a given scan to a next scan of the same line) or is inverted for every line by the AC driving method. Here, however, if attention is given to the voltage applied to the liquid crystal pixel under the condition where the non-linear resistance element exhibits asymmetric non-linear characteristics depending on the positive side and the negative side as described above, different voltages are eventually applied to the liquid crystal pixel since different voltages are applied to the non-linear resistance element depending on the positive side and the negative side.
As a result, flickering and deviation of ions in the liquid crystal causes the image to be printed on the pixel such as a residual image phenomenon and the display quality deteriorates greatly.
Also, Japanese Patent Application No. 181229/1989 discloses a method of enhancing the quality of display by compensating asymmetric non-linear characteristics. The driving method disclosed in application No. 181229/1989 will now be described with reference to FIGS. 12 and 11. As shown in FIG. 12, the feature of this driving method resides in that different offset voltages, i.e., Voff 3 and Voff 2 are applied to the scanning electrode depending upon writing and non-writing. Here, the offset voltages are set as described below. First, an element turn-on current during writing determined from the drive voltage and an element turn-off current during the non-writing are drawn on the diagram of voltage-current characteristics of a non-linear resistance element of FIG. 11. A voltage is found that corresponds to an intermediate point P1 of the voltage corresponding to the turn-on current between the positive side and the negative side, and is denoted as Voff 3. Similarly, a voltage is found that corresponds to an intermediate point P2 of the voltage corresponding to the turn-off current between the positive side and the negative side, and is denoted as Voff 2. Thus, the offset voltage is not simply applied but the offset voltages are independently set depending on the writing and the non-writing voltages, in order to realize the drive voltage that correctly corresponds to the voltage-current characteristics of the positive side and negative side of the non-linear resistance element.
The above-mentioned method of adjusting the offset voltage of the scanning signal is capable of preventing the quality of the display from deteriorating due to the asymmetric characteristics of the non-linear resistance element, but is not sufficient since the amplitude of the data signal remains constant and the transmission factor modulation range of the liquid crystal pixel for the write voltage is different depending on the positive side and the negative side.
There also exists a problem in the gradation display. FIG. 13 shows waveforms of data signals in the case when the gradation is displayed using pulse width modulation. The ratio of a period f in which the voltage is Vd1 to a period e in which the voltage is Vd2 is changed depending upon the gradation. Reference is made to FIG. 13 where a pixel is driven by a pulse having the same ratio of positive-side field to negative-side field. When the non-linear characteristics of the non-linear resistance element are greatly asymmetrical due to the polarity of the voltage, the transmission factors due to the positive-side field and the negative-side field become equal at a point g only as shown in FIG. 14 but are different in other transmission factor regions. Even in this case, therefore, it is not possible to sufficiently prevent the degradation of image quality caused by flickering and scorching. The object of the present invention is to provide a method of driving a liquid crystal display panel based on a pulse-width-modulation writing system of a high display quality which is free from problems caused by the pulse waveforms applied to the signal electrodes.